1. Field of the Invention
In extra-corporal lithotripsy, hyperthermy or in treatment of degenerated cells by means of elastic waves, locating the target, presents difficulties: through the high power elastic waves which effect the destruction of the target, either by heating or by imparting mechanical stresses can be easily focused by means of an acoustic system comprising a lens, a reflector or an emissive surface having a focus, the unavoidable differences in propagation speed of the elastic waves between the coupling media and the patient's body, which is of non homogeneous structure, give rise to refraction phenomena the effect of which is that the real focus position inside the body cannot be accurately determined from the knowledge of the theoretical position, such as defined by the acoustic system.
2. Description of the Prior Art
Several processes have been proposed for the location of the target. Those using X-rays have the drawback, besides their destructive character, are not able to show, for instance the location of certain gall-stones which are transparent to the X-rays. Ultrasonic echography is free of these drawbacks but presents at first sight a certain number of difficulties: the target can be reached only through a more or less narrow acoustic window; the destructive signals have a frequency too low for the echography and a power too high at least for extra-corporal lithotripsy.
If the destruction signals are produced by a piezoelectric ceramic, the ceramic has dimensions too large to be displaced at "real time" scanning rates, and anyway the focus must remain fixed during the treatment periods, thus excluding the use of the power transducer for the location during the treatment itself. The processes using an echography of the A type and measurements or determination effected without visual display are not sufficiently reliable for assuring the location control during the shootings, whereas this control is essential, since the target, specially when it concerns a stone, will move at the rate of the breathing and can even shift suddenly. In hyperthermy, a "real time" control is particularly essential and must be accurate, in order to avoid the destruction of healthy cells.
In the U.S. Pat. No. 4,617,931 filed on 26 Nov. 1984, the Applicant has described a visual location process which consists mainly of a real time echographic image formed of the target between the shootings during the treatment period, by means of an ultrasonic auxiliary beam, carrying out a scanning centered preferably in a symmetry plane of the main focused beam of destruction, wherein the plane of image formation thus passes through the main beam focus. Visual display of the target includes a mark showing the theoretical position of the focus, which is brought by the operator into coincidence with the image of the target by moving as a whole the two sources emitting the respective beams. The specification of said US patent is incorporated in this specification by way of reference as essential material thereof.
This process enables a permanent and accurate visual control of the location of the target during the shootings. The above-mentioned patent describes also an important improvement by which it can be verified, at any given moment that the ultrasonic destructive energy is effectively transmitted to the focus and that it is homogeneously distributed on the whole of the focal spot. To this effect, it discloses the use of the main source as an emitter of a fixed echographic beam, and of the auxiliary source as a receiver of the echoes formed by a reflection of said fixed beam, thus forming an image of the corresponding focal spot, by carrying out a scanning with the auxiliary source.